The field of the invention relates to rubber or elastomeric bearings (including laminated rubber bearings) used to support limited-movement between opposing loading members, all of which develop torques or forces more or less proportional to the extent of movement between the opposing members over a range, essentially an elastomeric spring effect. In some cases, the torques or forces required may exceed those ordinarily available, as exerted by humans for instance, and powered boosters have been necessary to achieve the desired extent of movement.
The following United States of America Patents are cited as references:
2,900,182Hinks3,228,673Hinks3,532,174Diamantides et al3,734,546Herbert, et al3,504,902Irwin6,524,007Hinks6,834,998Hinks5,794,753Kemper5,887,691Kemper5,967,283Kemper5,984,071Kemper4,607,382Dijkstra4,722,517Dijkstra5,178,357Platus5,310,157Platus5,390,892Platus5,669,594Platus
The following Canadian Patent is a further reference:
In the prior art, elastomeric bearings as disclosed in U.S. Pat. Nos. 2,900,182 and 3,228,673 include at least one, but usually multiple alternate laminations of metal or other strong inextensible material and rubber or elastomer usually bonded together. Lateral motions between succeeding metal laminations are permitted by shear strain within and parallel to the intervening rubber laminations. They can be made with layers in any shape, with apertures or not, and with various cross-sectional configurations, including truncated planar, conical, spherical, chevron-shaped or cylindrical layers.
All elastomeric bearings are used to separate and support opposed relatively moveable external loading members that bear upon the outer load-accepting layers or end pieces of the bearings that have load faces and are generally made of thicker metal. The opposing outer layers may be shaped to conform with and to seal with respect to their respective complemental loading members and to provide for keying to the latter for orientation and prevention of relative slipping.
When the external load faces of such a bearing are interposed between such complementally-contoured and opposed loading members, it can resist thrust, radial or combined forces normal to its layers, depending upon its configuration. Relative lateral movement between the opposed loading members, which may include pivoting about a normal axis as well as transverse or lateral shifting, results in a distribution of shearing movements between individual rubber layers.
An additional property of such a load-bearing bonded laminate stack that contains one or more apertures is the capability of preventing the lateral or transverse flow of fluids, i.e., liquids or gases, between the periphery of the laminate stack and an aperture, and making them essentially impervious even under substantial differential pressure. I.e., the space occupied by the bulk of the laminations between the opposing members is blocked against fluid penetration. U.S. Pat. Nos. 3,532,174, 3,734,546, 3,504,902, 6,524,007 and 6,834,998 exhibit the concept of rubber laminated bearings that seal against fluid flow, referred to here as bearing-seals. This sealing property is nevertheless irrelevant to the current invention.
As indicated above, elastomeric bearings and bearing-seals usually have the primary purpose of supporting loads and/or sealing between opposing members while permitting limited motion between said members, whether rotational or translational. Since that motion is the cumulative result of shear strain in the layers of elastomer itself, these devices usually develop negligible coulomb friction, but do exhibit an increasing resistive or reactive force or torque due to shear stress in the elastomer layers that accompanies the motion. This essentially linear spring effect can be described over the effective range by a number representing the rate of change of reactive force or torque acting against the displacement, i.e., its translational or torsional stiffness, i.e., spring rate.
In some cases, this stiffness is negligible in comparison to the forces or torques available to overcome them, and in others, it is a desirable effect. However, in situations where the high reaction forces or torques of elastomeric bearings exceed those of the means readily available to counteract them, those means have often been replaced, amplified, or supplemented by power booster means, which may be complex, expensive, unreliable, and weighty or otherwise undesirable.
This has often been particularly true for helicopter control systems. Laminated elastomeric bearings have frequently been made part of helicopter rotor hubs to retain each of the rotor blades against very high centrifugal forces while permitting their blade pitch, i.e., feathering, angles to be changed for control purposes. But except for small helicopters, it has been found that the forces required to change the pitch of the elastomeric-retained blades generally exceeds those available through human actuation of the pitch control sticks alone, and hydraulic boosters have conventionally been used to relieve the pilot from high control stick forces.
A similar situation, in principle, was faced by Kemper (U.S. Pat. No. 5,794,753, etc.) in a problem associated with the human-operated clutch of heavy trucks and other machinery. But rather than rely on conventional externally-powered actuators to help operate the clutch, he describes systems involving passive Bellville springs to accomplish that purpose. Bellville springs possess a non-linear force-displacement behavior that includes a region in which extended motion causes not a proportionally resisting force, but instead a force in the same direction and increasing with the motion. This is in effect defines a negative spring rate region of their force-displacement characteristic. Thus the Bellville spring's proportionally increasing force to separate the clutch plates helps the human act against the clutch springs that clamp the rotating clutch plates together.
Dijkstra (U.S. Pat. Nos. 4,607,382 and 4,722,517) and others referenced therein employ negative spring means to reduce the effective stiffness of loudspeaker cones and thereby lower their natural frequency.
Platus (U.S. Pat. No. 5,178,357, etc.) describes vibration-isolation platforms that employ a relatively stiff spring to support the weight of a mass placed upon the platform, together with negative spring means acting in parallel to reduce the effective local stiffness of the combined springs. This reduces the resonant frequency of the suspended spring-mass system so that results of higher frequency test vibrations applied to the mass are essentially unaffected by the suspension system.
Besides the inventions of Kemper, Dijkstra and Platus, many common extant devices employ elements that produce some characteristics of negative springs through involvement of sources of pushing or pulling forces. These force sources include passive springs of various kinds, such as coil, leaf, Bellville and Neg'ator springs, used in either tension or compression, and actuators powered by hydraulics, pneumatics, or electromagnetics, etc. The common quality of all these negative spring devices is that, with respect to some “center” position, they all exhibit a characteristic torque or force versus deflection behavior of urging movement farther away from the center over a range when initially deflected away from it, i.e., they exhibit a binary instability or “over-center” effect.
For instance, the common “snap-action” electrical switch, having many variations, often uses a pivoted compression spring that tends to force the associated contact assembly into one of two stable positions, either the “ON” or “OFF” state. When the spring is moved by a switch handle through its tightly compressed center position and goes over-center, the force of the spring on the contact assembly changes direction abruptly and causes it to quickly change states. A motion away from center causes a component of force to develop urging further movement in the same direction away from center. But in this case, there is no attempt to use this repelling effect to quantitatively compensate any positive spring continuously over a range of motion, but only to cause motion of the contact assembly as far as it will go in either direction.
On the other hand, Kemper and Dijkstra and Platus do calibrate their negative spring functionalities against the primary positive spring effects of their devices. Of these inventors, the first apparently uses the over-center effect only on one side of the force-centered position, while the others' apparatus operates on both sides of center.